A polymerizable compound having a liquid crystal phase provides a polymer having such a function as optical compensation through polymerization. This is because the alignment of the liquid crystal molecules is fixed through polymerization. For utilizing the function of the polymer, various polymerizable compounds have been developed. However, a sufficient function may not be attained only with a single compound. Under the circumstances, such an attempt has been made that a composition is prepared from plural kinds of polymerizable compounds, and the composition is polymerized (see, for example, JP-2006-307150-A and JP-2004-231638-A).
In the specification, the facts that a liquid crystal skeleton exhibits such an alignment state as a homogeneous alignment (horizontal alignment), a tilted alignment, a homeotropic alignment (vertical alignment) or a twisted alignment may be expressed as “having a homogeneous alignment”, “having a tilted alignment”, “having a homeotropic alignment” or “having a twisted alignment”, respectively. For example, a liquid crystal film having a homogeneous molecular alignment, i.e., a liquid crystal film oriented homogeneously, may be referred to as a liquid crystal film having a homogeneous alignment or a liquid crystal film of a homogeneous alignment.
A polymer having a homogeneous alignment can be used by combining, for example, with a ½ wavelength plate, a ¼ wavelength plate or a film having another optical function (see, for example, JP-2002-372623-A).
In the aforementioned purposes, a polymerizable liquid crystal material may be accumulated in some cases on a glass substrate, a glass substrate having a plastic thin film coated on the surface thereof (for example, an overcoating film formed on a color filter), a color filter substrate (see, for example, JP-2006-285014-A), or a plastic substrate. Examples of a material used as the plastic substrate include such polymers as TAC (triacetyl cellulose), polycarbonate, PET (polyethylene terephthalate) and a cycloolefin resin.
The inventors have found a polymerizable liquid crystal composition that can maintain the liquid crystal phase stably at room temperature to exhibit a uniform alignment property (see, for example, JP-2007-16213-A). However, in the case where the composition is changed in compositional ratio for changing the optical characteristics, particularly decreasing the birefringence (refractive index anisotropy Δn), a problem arises that the liquid crystal phase cannot be maintained. As another method for controlling the birefringence, a method of adding a compound having an aromatic ring in the short axis of the mesogen (a compound having a triptycene ring) has been proposed, but in the method, the addition of the compound in a large amount may cause crystallization, which brings about a problem in controlling the birefringence (see, for example, JP-2006-111571-A).